In this case the term “tractor vehicle or the like” is understood to mean a working vehicle in general, with which, during its operative use, high propulsion forces have to be produced by contact with the ground surface in order to generate a high thrust or traction force. As examples, agricultural tractors pulling a plow, or tractor shovels which push a loading shovel into a heap of material in order to take some of it up, may be mentioned. It is characteristic of such working processes that they take place at low driving speeds and are controlled by the driver via the control of the thrust. In known working vehicles the thrust is controlled, for example, by controlling the contact pressure in a slipping clutch, by specifying a speed difference in a hydraulic torque converter, or by means of some other torque-limiting element in the drivetrain, usually one that can be influenced by the driver.
The propulsion forces of the vehicle are produced by the contact of its tires with the ground. They depend on the normal force Fn at each drive wheel and on the coefficients of friction μ between the tires and the ground. In this, the frictional coefficient μ is particularly responsible for the forces that can be transmitted between the tires and the ground. However, the frictional coefficient itself depends on the material pairing (tire/ground) in each case and also on the slippage taking place at the wheel, i.e. the ratio between the actual speed of the vehicle over the ground and the theoretical speed on the basis of the wheel rotation speed. The known μ slip curves show that the μ value at first rises steeply with increasing slip and reaches a maximum, and then falls again along a flatter course. From this it follows that some slip is necessary in order to produce a propulsion force. The result of the μ value decrease after the maximum is that the excess drive torque not transmitted to the ground gives rise to an increasing, undesired acceleration, i.e. to wheel spin, which leads to increased tire wear and/or to damage of the road.
A method for controlling slip and a device are known from DE 102 19 270 C1. They are used with an agricultural utility vehicle which, for example, tows an agricultural implement. In this case devices are provided, which determine the slippage of the drive wheels and, if the actual slip is different from a specified nominal slip, regulate the drive torque by producing a defined combination of engine speed and gearbox transmission ratio in order to optimize the slip. A disadvantage of the known method and the known device is considered to be that in this regulation process a change of the engine speed also changes the drive torque, so that in some circumstances the power required in order to perform the given work is no longer available.